1. Field of the Invention
The present invention relates to a work machine such as a truck loader, a skid loader, etc. in which a traveling apparatus is driven via an HST (hydrostatic transmission) configured to drive an HST motor by an HST pump comprising a swash plate, variable displacement pump driven by an engine.
2. Description of the Related Art
As an example of a work machine including a traveling apparatus driven via an HST, there is known a machine including an HST pump comprising a swash plate, variable displacement pump, an HST motor connected in a closed circuit to this HST pump and a pair of speed-changing oil passageways and configured to be driven by discharge oil from the HST pump to drive a traveling apparatus, a charge circuit for supplementing an amount of oil from a charge pump to the speed-changing oil passageways, a charge relief valve for determining the circuit pressure of the charge circuit, a flushing valve for allowing escape of a portion of the work oil of the low-pressure side of the speed-changing oil passageways, a flushing relief valve incorporated in a flushing relief oil passageway for draining oil from the flushing valve, and a servo cylinder for controlling a swash plate of the HST pump so as to vary the discharge capacity of the HST pump according to the discharge rate from the charge pump (see JP 6-058411A).
In the case of a work machine configured such that its traveling apparatus is driven via an HST, if a significant load is applied to the HST motor during traveling, this load will be transmitted to the engine via the HST pump, so the rotational speed of the engine will be reduced, thus presenting the risk of stall of the engine. On the other hand, in the case of the construction described in JP 6-058411A, there is provided an anti-stall feature (anti-stall function). With this feature, when the rotational speed of the engine decreases, the rotational speed of the charge pump decreases also to reduce the discharge amount of this charge pump, whereby a control pressure of the servo cylinder is reduced, so that the swash plate angle of the HST pump is automatically adjusted so as to reduce the rotational speed of the HST pump. As a result, the load to the engine is reduced, thus providing an anti-stall feature (anti-stall function) for preventing engine stall.
In JP 6-058411A, as a method of improving the anti-stall feature for allowing the control pressure of the servo cylinder to drop more speedily in response to reduction in the rotational speed of the engine, it is conceivable to increase the amount of escape of the oil from the flushing relief valve or to increase the amount of escape of the oil from the charge relief valve. However, if the amount of the escape of the oil from the flushing relief valve is increased, due to the corresponding increase of the oil introduced to the flushing valve, e.g. malfunction of this flushing valve may occur. Further, there is a limit in increasing the amount of escape of the oil from the charge relief valve. Therefore, there is a limit in improvement of the anti-stall feature with these methods.
Further, in the case of an arrangement wherein the rotational speed of the engine is detected and a controller effects a control so that the control pressure of the swash plate of the HST pump is dropped speedily in response to application of excessive load to the engine, this will result in great complexity of the construction, thus inviting cost increase.
In view of the above-described problems, the object of the present invention is to provide a work machine that allows improvement of the anti-stall feature with simple arrangement.